1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventional image forming apparatuses including printers, copying machines, facsimile machines, and multi function printers (MFPs) involve an electrophotographic process where charging, exposing, developing, transferring, and fusing are performed in sequence. A charging roller charges the surface of a photoconductive drum. A light emitting diode (LED) head illuminates the charged surface of the photoconductive drum to form an electrostatic latent image. A developing roller rotates in contact with the photoconductive drum to supply toner to the electrostatic latent image to form a toner image. After transfer of the toner image onto a print medium, the photoconductive drum is cleaned of residual toner by a cleaning unit.
Dot population density in the present invention may be represented in terms of the ratio of the number of printed dots in a printable area to a total number of dots printable in the printable area. If an image having a low dot population density is printed repeatedly, a large percentage of the toner deposited on the developing roller remains unconsumed, so that the toner remaining on the developing roller will eventually be deteriorated. For solving this drawback, if an image has a low dot population density, the toner remaining on the developing roller after the development of the image, the residual toner is intentionally transferred to the photoconductive drum and then the toner on the photoconductive drum is collected as waste toner.
An image may not be necessarily uniform in the dot population density over the entire printable area. Even if an image has a high dot population density only in a limited area within the printable area, the average dot population density over the printable area may be low. A conventional image forming apparatus suffers from a problem in that if an image has a high dot population density only in a limited area within the entire printable area, the residual toner may not be collected thoroughly from the photoconductive drum and the residual toner will eventually deteriorate on the photoconductive drum. This causes spoiled images or poor print quality.